H-C-CH2CH2CCH3 + Criegee intermediate 4-oxopentanal
FIGURE 6.28 Oxidation of squalene to 6-methyl-5-hepten-2-one, acetone, geranyl acetone, and 4-oxo-pentanal (adapted from Fruekilde et al., 1998).
Some of the reactions, e.g., that of isoprene with OH and N03, were discussed earlier in this chapter. Table 6.26 summarizes some of the major products observed in the gas-phase reactions of several other biogenic hydrocarbons with OH and 03 (Atkinson, 1997a). These products are anticipated, based on the mechanisms described earlier in this chapter. As also expected, the yields of these major products generally do not account for 100% of the reactant lost, and there are a number of other products, including multifunctional species, that are also formed. As an example, the formation of more than 30 individual products has been observed from the reaction of a-pinene with 03 in air, some of which are unidentified, and the same is true for the A3-carene reaction (Yu et al., 1998). Products included hydroxy oxoacids, hydroxy dicarbonyls, and dicarbonyls. The formation of low-volatility products that form particles (e.g., Hoffmann et al., 1998; Jang and Kamens, 1999) is likely responsible for a significant fraction of the products missing from the gas phase. For example, Jang and Kamens (1999) have identified a variety of multifunctional oygenated products in aerosol particles from the a-pinene reaction with 03, including diacids such as 2,2-dimethylcyclobutane-l,3-dicarboxylic acid and pinic acid. These diacids have sufficiently low vapor pressures that they are believed to contribute significantly to the formation of particles in this system.
This is supported by studies of the aerosol composition in forested areas. For example, Kavouras et al. (1998) identified cis- and trans-pinonic acids as well as pinonaldehyde and nopinone in particles in a forest in Portugal. The diurnal variations of the pinonic acids and formic acid were similar, peaking in the afternoon as expected if they were formed by the reaction of 03 with a-pinene. On the other hand, the concentrations of pinonaldehyde, expected from the oxidation of a-pinene by OH, 03, and N03, and nopinone, from the oxidation of /3-pinene, were the smallest in the after-
TABLE 6.26 Some Products and Their Yields Observed in the Gas-Phase Reactions of Some Biogenic Hydrocarbons with OH and 03a
OH reaction yield O, reaction yield
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